1. Field of the Invention
The invention relates to an antenna coil that is used, for example, in a keyless entry system for motor vehicles.
2. Description of Related Art
Previously, techniques were known for electronic key systems for motor vehicles. The techniques include an antenna coil in an IC card (a transceiver on a portable side) as an electronic key. The antenna coil is electromagnetically coupled to the antenna coil of an IC card in a card reader (a transceiver on an external side) of a motor vehicle. The antenna of the card reader on the motor vehicle side transmits an inquiry signal in the air. When the antenna coil of the IC card receives the inquiry signal, the IC card sends out an ID code signal characteristic of each vehicle as a response signal from the antenna coil. The card reader reads the ID codes from the IC card to compare the ID codes with ID codes beforehand on the vehicle side. When both of the codes are matched, some operations such as a door lock or start-up of an engine are performed, which is a known technique, for example, in Japanese Patent Publication (Laid Open) Hei 8-274682.
In recent years, in order to obtain nondirectional reception sensitivity, a three-axis antenna coil having three coils, where a direction orthogonal to the others is determined to be an axis, have been proposed. As for the three-axis antenna coil, considering downsizing and portability of the device, the three coils have to be positioned as effectively as possible, taking up as small an area as possible.
As examples, Japanese Patent Publications (Laid Open) 2003-92509 and 2004-32754 disclose that the X axis coil and Y axis coil are wound around a spool of a core, with the insulating spool located so as to encircle the X axis and Y axis coils, where a z axis coil is wound around the spool.
Japanese Patent Publication (Laid Open) 2003-249816 discloses the antenna coil, where concave portions for a cross winding are formed in four sides of a core of a flat plate, outer circumference grooves for the winding are provided in an outer circumference of the concave portions for the winding, the X axis coil and Y axis coil are wound in the concave portions for winding, and the Z axis coil is wound in the outer circumference grooves for the winding.
The three axis antenna coil of the prior art provides concave portions such as grooves in the core or spool mounted around thereof. In the concave portions, the coils are wound and held. However, in order that the coils do not come off the core or spool, the depth of the concave portions is larger than the thickness of layers which the wound coils constitute. In other words, an overhang is formed that overhangs from the outer surface in the circumference of the concave portions, which is disadvantageous in making the antenna coil smaller or thinner. Since a shape of the core or spool is complicated to hold the coils in the concave portions, it takes time and labor to wind the coils around the core or spool, which produces a problem of incurring increased manufacturing cost.
According to the inventions described in Japanese Patent Publications 2003-92509 and 2004-32754, electrodes (terminals), to which lead terminals for the coils are connected, are mounted to an upper and a lower surface of the spool, which is disadvantageous in reducing thickness of an antenna coil because the thickness of the antenna coil increases. Since a portion connecting the lead terminal and the electrode overhangs from the spool, the electrode is subject to coming off or being damaged when an impulse is applied.
Furthermore, when this kind of antenna is mounted on a printed board, if a wiring pattern formed on the printed board or a conductor such as an electronic component is provided beneath the antenna coil, an inductance of the coil and a value of Q are lowered, which deteriorates sensitivity of the antenna. Because of this, an empty region in which there is no wiring pattern or electronic component is formed on the printed board, and in the empty region, an antenna coil is placed. However, in this case, it is difficult to reduce an area for mounting the antenna coil.
For the foregoing reasons, there is a need for an antenna coil that has a simple structure and a high sensitivity, and in addition has a small thickness.
Furthermore, for the foregoing reasons, there is another need for an antenna coil that has can reduce a mounting area on a printed board where the antenna coil is provided on the printed board.